A Primary Chilled Water Pump in Primary/Secondary pumping HVAC system application is designed to maintain a constant flow through cooler of the chiller. The Primary/Secondary pumping technique decouples the “primary” production loop from the “secondary” distribution loop. This allows chillers to obtain constant design flow and operate properly while allowing the rest of the system (Secondary Pumping) to vary in flow.

In Primary Chilled water system, the pumps is sized to handle the chiller cooler design flow rate at a discharge pressure just high enough to circulate the water through the cooler of the chiller and the rest of the primary piping loop.

The primary pump flow is traditionally controlled by throttling valves or balancing valve on the discharge side of the pump. The pumps are often oversized due to safety margin in the designs. By adding losses to the pumping circuit with the throttling valve, the proper design flow rate can be established.

The primary pump before retrofitting is controlled by throttling valve which is partially closed.

VSD were added to the primary pumping to replace the throttling valves. The throttling valve will be FULLY OPENED; VSD will act as “electronic valve” to bring the flow back to design condition. There are two possible control methods:

1) Using the flowmeter. Because the desire design flow rate is known and is constant, a flow meter can be installed at the discharge of each chiller and the analogue output from the flowmeter controlled the pump directly.

2)Using local speed determination. The speed was manually decreased until design flow rate is achieved. Differential pressure sensors can be used by locating across each chiller. Differential pressure across a non modulating load also measure flow.

In this application, differential pressure sensors were being used.